Thinly slice 1 shallot then add to a bowl and cover with cold water; set aside.
Heat a cast-iron skillet over medium heat for 5 minutes.
Pat steak dry then season generously with salt (at least 1 teaspoon).
Add steak to skillet and cook, without touching, until steak naturally releases from pan and is golden brown, about 4 minutes, flip and continue to cook until golden brown on second side and an instant-reader thermometer registers 125ºF for medium-rare, 135ºF for medium, or 145ºF for well done.
Transfer steak to a plate, tent loosely with foil and let rest 10 minutes.
Transfer steak to a cutting board and cut into thin strips, against the grain. Cut strips into a small dice; transfer diced steak to a large mixing bowl.
Drain shallot and add to bowl with steak along with ½ bunch chopped cilantro, ⅓ cup chopped mint, 1 tablespoons minced lemongrass, 1 tablespoon minced kaffir leaves, 2 teaspoons grated ginger, and ½ teaspoon pepper flakes.
Whisk together 2 teaspoon fish sauce and ½ teaspoon anchovy paste in a small bowl. Add fish sauce mixture and juice of 1 lime juice to salad; toss to combine. Stir in 1 tablespoon toasted rice powder, if using. Season to taste with additional salt, pepper flakes and lime juice.
Transfer salad to a large serving bowl or platter and top with sliced chiles, if using. Serve with sesame rice and large lettuce leaves.